You do need to see a doctor. Even though it may drain it may not drain thoroughly and can redevelop. Antibiotics are almost always given for infection. Some people (many) are "carriers" of MRSA and do not realize it. Forcing/popping it open to get it to drain is not a good idea. Most always it will open and drain on its own but there are times when it has to be opened, drained and then packed open to heal.
torie, it is not the water you drink, it is the water in which you wash: shower, pool, river and so on. all those can be contaminated. as a result a small mosquito bite or an irritated area can get infected.
if you do not see a dermatologo you risk repeated infections. some of the areas (face and the head) can be very dangerous too if affected. dr1ers can advice you an english speaking dermatologo (i know a decent one in POP, others can point you good doctors in different cities).
i suggest you try some antibiotic cream (most likely that's what you'd get anyways). maybe fucidin? it costs about 500-700 pesos. there are some cheaper creams with antibiotics too, like derma3 but they also contain anti fungal meds. clean the area, apply cream twice a day. use talc to keep general area dry. use only cotton for now and at home take off the shirt altogether.